Paid ad campaigns can be run for any length of time - we usually run the first campaign for 30 days to collect data and find opportunities to improve the campaign performance and lower costs.
Strategically, we first organically optimize the website and track its rankings on Google Analytics for at least 30 days. Then, for competitive keywords that are not gaining rankings we can set up a targeted pay per click campaign. We recommend both Google AdWords and the Microsoft Ad Center (Bing and Yahoo)
Our experience, after many years of reviewing analytics reports, has shown that Google may get 60 to 90 percent of online queries. Bing and Yahoo get the majority of the rest, depending on some demographic issues. We have also learned that the ROI on these two search engines is much greater than Google, which tends to see click prices bid up to unreasonable levels. So, adding the Microsoft Ad Center program is usually worth the extra effort.
The search engines make it very easy to spend a lot of money on irrelevant clicks. To be cost efficient, setting up a campaign is a multi step process. Quality Scores are the most important issue, followed by the development of a negative keywords list. Optimizing these usually takes a few rounds of adjustments to both the campaigns and the actual landing pages. We always strive for the highest Quality Scores to get lower click costs and better conversions.
We set up these accounts in the clients name with their credit card info, give them full administrative access, and train them on how manage their campaigns. For a monthly fee we can manage their campaigns and send them weekly reports.
Here's an interesting article from Google about the cost of AdWords